Method of making a circuitized substrate

ABSTRACT

A circuitized substrate and a method of making the circuitized substrate are provided. The circuitized substrate includes a substrate having a conductive pad thereon. A first layer of solder enhancing material is positioned on the conductive pad, the first layer of solder enhancing material includes a first region and a second region positioned relative to the first region. A solder member is positioned on the first region of the first layer of solder enhancing material. A second layer of solder enhancing material is positioned on the solder member and on a portion of the second region of the first layer of solder enhancing material. The circuitized substrate may be used in the fabrication of an electronic package.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/205,136,filed Jul. 25, 2002 now U.S. Pat. No. 6,818,988.

FIELD OF THE INVENTION

This invention relates generally to electronic packaging, and moreparticularly to a method for mounting a solder member on a conductivepad of a substrate to make a circuitized substrate.

BACKGROUND OF THE INVENTION

The use of tin-lead based solders for electronic packaging is wellestablished. Market and legislative pressures are accelerating thereduction, and perhaps elimination, of lead from solders used inelectronics packaging. Unfortunately, lead-free alloys typically havepoorer wetting characteristics than lead based solder alloys.

Tin-lead solder is widely used in the electronics industry forelectrical and mechanical interconnections between devices and chipcarriers and between chip carriers and printed wiring boards. Theinterconnection between a chip carrier and a printed wiring board ofteninvolves a ball grid array (BGA) interconnection.

The manufacture of a BGA interconnection on a chip carrier usuallyinvolves the attachment of solder members, for example, spheres, toconductive (and solderable) pads on the chip carrier. Solder paste isanother method used to form the BGA interconnection. Regardless of theprocess used, a solder enhancing material such as a solder flux isusually used to enhance solderability. Solder flux serves two importantfunctions. First, it provides an adhesive property that maintains soldermembers in position after they are positioned on conductive pads until amechanical interconnection is made by soldering. Flux also functions toremove oxides from the metallic surfaces of both the solder members andconductive pads, promoting a robust metallurgical bond, during reflowand solidification, between solder members and conductive pads.

Solder fluxes used in the electronics industry vary depending onapplication method and whether the flux is water soluble or no-clean. Awater soluble flux is a flux that leaves aqueous residue behind that canbe cleaned with water after processing. A no-clean flux is a flux thatleaves non-ionic residues behind that are typically left on the partsfor further processing. Either type of flux can be sprayed, screened, ortransferred by pins onto the conductive pads to be soldered. Sometimessolder members are dipped into flux and then positioned on conductivepads.

During the heat up and reflow process used to make the interconnection,flux chemically reacts with oxides on the surface of the soldermembers/conductive pads and oxygen in the reflow oven environment.Increasing the quantity of flux used, usually improves the solderwetting but can negatively affect the process yield because the soldermembers may be able to “float” up and off the conductive pads and/orcreate electrical bridges (shorts) with neighboring interconnectionscomprising the BGA.

Typically, tin-lead solders exhibit robust wetting in air reflow ovens,however the wetting of lead-free solders is negatively impactedespecially by the presence of oxygen during reflow. Substitution ofnitrogen for air in the lead-free reflow process can be very expensiveand cumbersome.

As a result, there exists in the industry a need for a more reliable,less costly method of making an interconnection between a solder member,particularly a lead-free solder member, and a conductive pad of asubstrate to make a circuitized substrate which overcomes thedisadvantages of the known method and structure.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to enhance the art ofelectronic packaging.

Another object of the present invention is to provide a circuitizedsubstrate for use in the manufacture of an electronic package includinga substrate having a conductive pad thereon, a first layer of solderenhancing material positioned on the conductive pad, a solder memberpositioned on a portion of the first layer of solder enhancing material,and a second layer of solder enhancing material positioned on the soldermember and on exposed portions of the first layer of solder enhancingmaterial.

Yet another object of the present invention is to provide a method ofmaking a circuitized substrate having a substrate with a conductive padthereon, a first layer of solder enhancing material positioned on theconductive pad, a solder member positioned on a portion of the firstlayer of solder enhancing material, and a second layer of solderenhancing material positioned on the solder member and on exposedportions of the first layer of solder enhancing material.

Another object is to provide such a circuitized substrate and methodthat are both adaptable to mass production, thus assuring lower costs.

According to one aspect of the invention there is provided a circuitizedsubstrate comprising a substrate having at least one conductive padthereon, a first layer of solder enhancing material positioned on the atleast one conductive pad, the first layer of solder enhancing materialincluding a first region and a second region positioned relative to thefirst region, a solder member positioned on the first region of thefirst layer of solder enhancing material, and a second layer of solderenhancing material positioned on the solder member and on a portion ofthe second region of the first layer of solder enhancing material.

According to another aspect of the invention there is provided acircuitized substrate comprising a substrate having at least oneconductive pad thereon, the at least one conductive pad including afirst portion and a second portion positioned adjacent the firstportion, a first layer of solder enhancing material positioned on thefirst portion of the at least one conductive pad, the first layer ofsolder enhancing material including a first region and a second regionpositioned relative to the first region, a solder member positioned onthe first region of the first layer of solder enhancing material, and asecond layer of solder enhancing material positioned on the soldermember, on the second region of the first layer of solder enhancingmaterial, and on the second portion of the at least one conductive pad.

According to yet another aspect of the invention there is provided amethod for making a circuitized substrate comprising the steps ofproviding a substrate having at least one conductive pad thereon,applying a first layer of solder enhancing material on the at least oneconductive pad, the first layer of solder enhancing material including afirst region and a second region positioned relative to the firstregion, positioning a solder member on the first region of the firstlayer of solder enhancing material, and applying a second layer ofsolder enhancing material on the solder member and on a portion of thesecond region of the first layer of solder enhancing material.

According to still yet another aspect of the invention there is provideda method for making a circuitized substrate comprising the steps ofproviding a substrate having at least one conductive pad thereon, the atleast one conductive pad including a first portion and a second portionpositioned adjacent to the first portion, applying a first layer ofsolder enhancing material on the first portion of the at least oneconductive pad, the first layer of solder enhancing material including afirst region and a second region positioned relative to the firstregion, positioning a solder member on the first region of the firstlayer of solder enhancing material, and applying a second layer ofsolder enhancing material on the solder member, on the second region ofthe first layer of solder enhancing material, and on the second portionof the at least one conductive pad.

The above objects, advantages, and features of the present inventionwill become more readily apparent from the following detaileddescription of the preferred embodiments as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a much enlarged partial sectional view, in elevation, of oneembodiment of the circuitized substrate of the present inventionillustrating a substrate having a conductive pad thereon, a first layerof solder enhancing material positioned on a portion of the conductivepad, a solder member positioned on a portion of the first layer ofsolder enhancing material, and a second layer of solder enhancingmaterial positioned on the solder member, on the exposed portion of thefirst layer of solder enhancing material, and on the conductive pad.

FIG. 2 is a much enlarged partial sectional view, in elevation ofanother embodiment of the circuitized substrate of the presentinvention, illustrating a substrate having a conductive pad thereon, afirst layer of solder enhancing material positioned on the conductivepad, a solder member positioned on a portion of the first layer ofsolder enhancing material, and a second layer of solder enhancingmaterial positioned on the solder member and a portion of the firstlayer of solder enhancing material.

FIG. 3 is a process flow diagram showing the method of making thecircuitized substrate according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A circuitized substrate 10 illustrating one embodiment of the presentinvention is shown in FIG. 1. The circuitized substrate includes asubstrate 12 having a conductive pad 14 thereon. Conductive pad 14 has afirst portion 16 and a second portion 18 positioned adjacent to thefirst portion. A first layer of solder enhancing material 20 ispositioned on first portion 16 of conductive pad 14. First layer ofsolder enhancing material 20 includes a first region 22 and a secondregion 24 positioned relative to the first region. A solder member 26 ispositioned on first region 22 of first layer of solder enhancingmaterial 20. A second layer of solder enhancing material 28 ispositioned on solder member 26, on second region 24 of first layer ofsolder enhancing material 20, and on second portion 18 of conductive pad14.

Substrate 12 can comprise a chip carrier or any suitable substrate onwhich a solder member is to be positioned for electrical interconnectionpurposes. Suitable substrates that can be used in this invention may becomprised of epoxy resins filled with glass, ceramics, and silicon.Conductive pad 14 can be electrically connected to conductive throughapertures 15 within substrate 12 to make electrical connections to otherelectronic devices.

First layer of solder enhancing material 20 and second layer of solderenhancing material 28 can both be comprised of organic or inorganicacids in a solvent system. Examples of acids suitable for use in thisinvention are amine hydrochlorides and carboxylic acids. Suitablecommercially available solder enhancing materials are Alpha WS600 fromAlpha Metals, 600 Route 440, Jersey City, N.Y., and Kester SE-CURE 9750from Kester Solder, 515 East Toucy Ave., Des Plaines, Ill. Examples ofsuitable solvents are water, alcohols and glycols. Examples of glycolsthat can be used as solvents in this invention are polyalkyl glycol,hexylene glycol, and diethylene glycol. The solder enhancing materialsmay include surfactants and thickeners to improve its performance. Firstlayer of solder enhancing material 20 includes a thickness of from about0.0002 inches to about 0.005 inches. This thickness is important becauseif first layer of solder enhancing material 20 is too thin there may notenough material to hold solder member 26 in place and there may not beenough active ingredients present to ensure good wetting of the soldermember on conductive pad 14 during heat-up and reflow. If first layer ofsolder enhancing material 20 is too thick the solder member may beeasily displaced from its desired location during processing.

Solder member 26 preferably comprises a lead-free solder and is selectedfrom the group of metals consisting of tin, silver, copper, antimony,bismuth, indium, zinc, and alloys thereof. For example, one solder whichcan be used in this invention is a lead-free solder having about 3.8 wt.% silver, about 0.7 wt. % to about 0.9 wt. % copper, with the remaindertin. Solder member 26 can be substantially in the shape of a sphere,cylinder, or rectangle. First layer of solder enhancing material 20includes adhesive properties so that solder member 26 can besubstantially held in place on conductive pad 14 by first region 22 ofthe first layer of solder enhancing material. First region 22 of firstlayer of solder enhancing material 20 is defined by the contact areasubstantially under solder member 26 and is dependent on the size andshape of the solder member. The second layer of solder enhancingmaterial 28 includes a thickness of less than about 0.001 inches.

In the absence of solder enhancing material, a lead-free solder memberpositioned on a conductive pad, such as copper, when reflowed in anenvironment including oxygen, for example in air, will have a tendencyto not completely wet the conductive pad. Positioning lead-free soldermember 26 on conductive pad 14 with only a first layer of solderenhancing material 20 is not sufficient to result in complete wetting ofthe conductive pad in a reflow environment including oxygen.Substituting a nitrogen rich, essentially oxygen free, environment wouldresult in complete wetting of the lead-free solder member to conductivepad 14, however, this requires substantial expense and specializedreflow ovens. This is impractical and very costly in a large scalemanufacturing environment. Significantly, second layer of solderenhancing material 28 in combination with first layer of solderenhancing material 20, as taught in this invention, eliminates the needfor use of a costly nitrogen rich, substantially oxygen free reflowenvironment and leads to substantially complete wetting of conductivepad 14 by solder member 26 during reflow of the solder member. Thesecond layer of solder enhancing material 28 enhances wetting in manyways. Since it is positioned on solder member 26 and second portion 18of conductive pad 14 it can react with the oxygen present in the reflowenvironment during reflow to prevent oxygen from oxidizing solder member26 and second portion 18 of conductive pad 14. Second layer of solderenhancing material 28 also provides a physical barrier betweenenvironmental oxygen, solder member 26 and conductive pad 14 preventingoxidation. Furthermore, first and second layers of solder enhancingmaterials 20 and 28, respectively, can react with native oxides onsolder member 26 and conductive pad 14 to consume these oxides andresults in substantially complete wetting of the conductive pad by thesolder member during reflow.

FIG. 2 illustrates another embodiment of a circuitized substrate 110 ofthe present invention where solder member 126 can substantially wetconductive pad 114 during reflow of the solder member. Circuitizedsubstrate 110 includes a substrate 112 having at least one conductivepad 114 thereon. A first layer of solder enhancing material 120 includesa first region 123 and a second region 125, positioned relative to thefirst region. Solder member 126 is positioned on first region 123 offirst layer of solder enhancing material 120. A second layer of solderenhancing material 128 is positioned on solder member 126 and on aportion 130 of second region 125 of first layer of solder enhancingmaterial 120.

Referring to FIG. 3, a method 50 of making a circuitized substrate isshown. The first step 52 in this method is providing a substrate havingat least one conductive pad thereon, the conductive pad including afirst portion and a second portion positioned adjacent to the firstportion. The substrate has been previously described in detail above.

Step 54 includes applying a first layer of solder enhancing material onthe conductive pad, the first layer of solder enhancing materialincluding a first region and a second region. The first layer of solderenhancing material may be applied by pin transferring, dipping,stenciling, or screening on only the first portion of the conductivepad, or on the first and second portions of the conductive pad tosubstantially cover the conductive pad. Pin transfer or screening allowsfor a uniform thickness of solder enhancing material to be deposited onthe conductive pad. The quantity of solder enhancing material useddirectly influences the amount of coverage and robustness of wetting ofa solder member on the conductive pad during reflow. By controlling thearea of the conductive pad and the amount of solder enhancing materialused, final coverage is controlled. Applying the thickness of firstlayer of solder enhancing material, as described above, between about0.0002 inches and about 0.005 inches will assist in complete wetting ofthe conductive pad on which the solder member is to be positioned.Increasing the thickness of first layer of solder enhancing materialabove about 0.005 inches can cause the solder member to float or moveduring processing and can lead to bridging of solder between adjacentconductive pads. This can cause electrical shorts. The first layer ofsolder enhancing material can be an organic or inorganic acid in asolvent system, as described above.

Step 56 includes positioning a solder member on the first region of thefirst layer of solder enhancing material. The solder member is describedin detail above. The solder member can be held in place by a template.The solder enhancing material is tacky and tends to hold the soldermember in place and the first layer of solder enhancing material is thinenough so that surface tension can provide resistance to movement of thesolder member during processing.

Step 58 includes applying a second layer of solder enhancing material onthe solder member. This step can further include applying a second layerof solder enhancing material on a portion of or on all of the secondregion of the first layer of solder enhancing material, if additionalmaterial is desired. This will depend on the lead-free solder alloy tobe used. The second layer of solder enhancing material can also beapplied to the second portion of the conductive pad if additional fluxis required with the lead-free solder being used. Applying the secondlayer of solder enhancing material comprises spraying. Spraying isaccomplished at a temperature of from about 68 degrees Fahrenheit (° F.)to about 73° F., at a pressure of from about 10 pounds per square inch(psi) to about 500 psi for less than about a minute. It is important tocontrol the amount of the second layer of solder enhancing materialapplied to the solder member to give the desired thickness in asubstantially uniform manner and not to dislodge the solder member. Thiscan best be accomplished by spraying the second layer of solderenhancing material onto the solder member. A dipping or screeningprocess, if used to apply the second layer of solder enhancing material,will not yield these advantages. The second layer of solder enhancingmaterial can be an organic or inorganic acid in a solvent system, asdescribed above, but is not limited to the same acid or solvent systemas the first layer of solder enhancing material.

Step 60 includes heating the solder member, the conductive pad, and thefirst and second layers of solder enhancing material to substantiallyclean the conductive pad and the solder member with the first and secondlayers of solder enhancing material. Heating is performed at atemperature of from about 70° C. to about 260° C. for a time of fromabout 7 minutes to about 15 minutes. Heating also melts and reflows thesolder member and upon cooling bonds the solder member electrically andmechanically to the conductive pad to form an interconnection. Fluxresidues can remain on the surface of the substrate or can be removedwith a solvent by spraying or immersion of the substrate in the solvent.The circuitized substrate is now ready for further assembly.

What has been shown and described are at present considered thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications can be madetherein without departing from the scope of the invention as defined bythe appended claims.

1. A method of making a circuitized substrate comprising the steps of: providing a substrate having at least one conductive pad thereon; applying a first layer of solder enhancing material on said at least one conductive pad, said first layer of solder enhancing material including a first region and a second region positioned relative to said first region; positioning a solder member on said first region of said first layer of solder enhancing material; and applying a second layer of solder enhancing material on said solder member and on a portion of said second region of said first layer of solder enhancing material, and wherein said step of applying said first layer of solder enhancing material comprises pin transferring, dipping, stenciling, or screening.
 2. The method of making the circuitized substrate of claim 1 wherein said step of applying said second layer of solder enhancing material comprises spraying.
 3. The method of making the circuitized substrate of claim 1 wherein said spraying is performed at a temperature of from about 68° F. to about 73° F., at a pressure of from about 10 psi to about 500 psi for less than about one minute.
 4. The method of making the circuitized substrate of claim 1 further including the step of heating said solder member, said at least one conductive pad, and said first and second layers of solder enhancing material to substantially clean said at least one conductive pad and said solder member with said first and second layers of solder enhancing material, and wherein said heating step is performed at a temperature of from about 70° C. to about 260° C.
 5. The method of claim 4 where said heating step is performed for a time of from about 7 minutes to about 15 minutes.
 6. A method of making a circuitized substrate comprising the steps of: providing a substrate having at least one conductive pad thereon, said at least one conductive pad including a first portion and a second portion positioned adjacent said first portion; applying a first layer of solder enhancing material on said first portion of said at least one conductive pad, said first layer of solder enhancing material including a first region and a second region positioned relative to said first region; positioning a solder member on said first region of said first layer of solder enhancing material; and applying a second layer of solder enhancing material on said solder member, on said second region of said first layer of solder enhancing material, and on said second portion of said at least one conductive pad, and wherein said step of applying said first layer of solder enhancing material comprises pin transferring, dipping, stenciling, or screening.
 7. The method of making the circuitized substrate of claim 6 wherein said step of applying said second layer of said solder enhancing material comprises spraying.
 8. The method of making the circuitized substrate of claim 7 wherein said spraying is performed at a temperature of from about 63° F. to about 73° F., at a pressure of from about 10 psi to about 500 psi for less than about one minute.
 9. The method of making the circuitized substrate of claim 6 further including the step of heating said solder member, said at least one conductive pad, and said first and second layers of solder enhancing material to substantially clean said at least one conductive pad and said solder member with said first and second layers of solder enhancing material, and wherein said heating step is performed at a temperature of from about 70° C. to about 260° C.
 10. The method of claim 9 where said heating step is performed for a time of from about 7 minutes to about 15 minutes.
 11. A method of making a circuitized substrate comprising the steps of: providing a substrate having at least one conductive pad thereon; applying a first layer of solder enhancing material on said at least one conductive pad, said first layer of solder enhancing material including a first region and a second region positioned relative to said first region; positioning a solder member on said first region of said first layer of solder enhancing material; and applying a second layer of solder enhancing material on said solder member and on a portion of said second region of said first layer of solder enhancing material, and wherein said first and second layers of solder enhancing material each comprise a solution consisting essentially of amine hydrochlorides and carboxylic acids in a solvent system.
 12. A method of making a circuitized substrate comprising the steps of: providing a substrate having at least one conductive pad thereon, said at least one conductive pad including a first portion and a second portion positioned adjacent said first portion; applying a first layer of solder enhancing material on said first portion of said at least one conductive pad, said first layer of solder enhancing material including a first region and a second region positioned relative to said first region; positioning a solder member on said first region of said first layer of solder enhancing material; and applying a second layer of solder enhancing material on said solder member, on said second region of said first layer of solder enhancing material, and on said second portion of said at least one conductive pad, and wherein said first and second layers of solder enhancing material each comprise a solution consisting essentially of amine hydrochlorides and carboxylic acids in a solvent system.
 13. A method of making a circuitized substrate comprising the steps of: providing a substrate having at least one conductive pad thereon, said at least one conductive pad including a first portion and a second portion positioned adjacent said first portion; applying a first layer of solder enhancing material on said first portion of said at least one conductive pad, said first layer of solder enhancing material including a first region and a second region positioned relative to said first region; positioning a solder member on said first region of said first layer of solder enhancing material; and applying a second layer of solder enhancing material on said solder member, on said second region of said first layer of solder enhancing material, and on said second portion of said at least one conductive pad, and wherein said first layer of solder enhancing material includes a thickness of from about 0.0002 inches to about 0.005 inches.
 14. A method of making a circuitized substrate comprising the steps of: providing a substrate having at least one conductive pad thereon, said at least one conductive pad including a first portion and a second portion positioned adjacent said first portion; applying a first layer of solder enhancing material on said first portion of said at least one conductive pad, said first layer of solder enhancing material including a first region and a second region positioned relative to said first region; positioning a solder member on said first region of said first layer of solder enhancing material; and applying a second layer of solder enhancing material on said solder member, on said second region of said first layer of solder enhancing material, and on said second portion of said at least one conductive pad, and wherein said first layer of solder enhancing material includes a thickness of from about 0.0002 inches to about 0.005 inches. 